This invention is related to a brake pressure control apparatus for automotive vehicles, with an anti-locking control system (ABS) and/or with a traction slip control system (TSS), which is operated by fluid pressure. Such apparatus includes a master cylinder, one or more wheel cylinders, each for a corresponding wheel brake, a pressure modulator for the modulation of the hydraulic pressure within the wheel cylinders during the brake pressure control mode, at least one motor-driven pump for the generation of a fluid pressure, and an electronic controller.
An example of brake systems of this type is described in published but unexamined German patent application No. 3,731,603.
Brake systems of this type are equipped with an anti-locking control system and/or with a traction slip control system. They are furnished with one or with two auxiliary pumps which deliver fluid under pressure into a pressure modulator during the anti-locking control mode to reincrease the pressure in the wheel cylinders in the anti-lock control mode.
The pressure modulator is comprised of a plurality of hydraulic valves. In particular, inlet and outlet valves are provided in said pressure modulator which are connected to the associated wheel cylinder.
The driver stage of the electronic controller pilots said inlet and outlet valves in accordance with the control algorithm which is installed in the controller. A pressure reduction phase is generated in the wheel cylinder when the outlet valve is open and the inlet valve is closed. When the inlet valve is closed and the outlet valve is open, a pressure stabilization phase comes about. When the inlet valve is open and the outlet valve is closed, a pressure rebuilding-up phase commences.
Furthermore, hydraulic circuits have been described in which the inlet valve has a restrictive effect. See the German patent application No. 3,919,842.1. in this respect.
In that patent application, a hydraulic brake system for a vehicle is described which is equipped with an anti-locking control system and which is comprised of a master cylinder, a fluid pressure accumulator, at least one wheel brake which is in communication with the master brake cylinder through a brake line and is in communication with the fluid pressure accumulator through a return line. An electromagnetically actuated outlet valve is inserted in the return line which shuts off the return line in its normal position and releases the return line in its switching position. A restriction valve is also incorporated in the brake line and which is furnished with two switching positions, in a first switching position, an unrestricted connection exists between the wheel brake and the master brake cylinder, and in a second switching position, a restricted connection exists between the wheel brake and the master brake cylinder.
A motor driven pump aspirates fluid from the accumulator and delivers it through a pressure line to the restriction valve. A sensor monitors the angular velocity of the wheel to be braked, and an electronic evaluation unit evaluates the sensor signal and generates switching signals for the pump drive and for the energization of the outlet valve.
It is proposed in the aforementioned German patent application that the restriction valve is connected by means of a control line to the outlet of the pump so that the pressure at said outlet of the pump shifts the restriction valve from the first into the second switching position.
In the system described in the German patent application no. P 3919842.1, a restrictor becomes effective in the inlet line going to the wheel cylinder during the control mode. In the course of the control mode, pressurized fluid is conveyed by the pumps into the wheel cylinder through the aforementioned restrictor. A pressure rebuild-up in the wheel cylinder comes about.
The pressure reduction is attained by the opening of an outlet valve. The outlet valves in question are constituted by electromagnetically actuatable valves, closed when de-energized (NC valves). According to the teachings of the state of the art, the NC valves are piloted by an output signal of the electronic controller included in the anti-locking control system.